Electrochromic devices that provide variable transmittance of light can have application in windows, mirrors, and various display devices. Commercially available electrochromic devices are most commonly composed of multiple layers containing at least: i) two conductive electrodes, often coated or printed onto glass or other transparent substrates, ii) an inorganic and/or organic chromophore layer, and iii) a liquid or gel electrolyte layer. The major disadvantage of such a multilayer system is its complexity and the requirement that expensive sputtering or chemical vapor deposition technologies be used in its manufacture. Furthermore, it is not easy to prepare large surface area devices using these technologies.
Another disadvantage is that large electrochromic devices employing liquid electrolytes sandwiched between glass supports can develop large hydrostatic forces, which can break or separate from the glass supports. If the glass supports break, the liquid electrolytes can spill. In addition, systems prepared using a gel/liquid electrolyte layer are susceptible to deactivation when laminated at high temperatures.
The use of gel electrolytes can mitigate spillage issues, but gels do not provide adhesion between the substrates, and hence cannot be used with thin (weight-conserving) glass substrates.
Conductive polymers have also been employed for dynamic glazing applications, but they suffer from high cost and poor processibility.
Solid-state, single-layer electrochromic devices based on polyvinylbutyral (PVB) are known. These systems can be prepared using solution methods, but when such compositions are prepared using melt-processing technologies, the electrochromic response of the device is retarded. Systems prepared using solution methods are also susceptible to deactivation when laminated at high temperatures.
In-situ polymerization of a mixture comprising a polymerizable monomer, an electrochromic compound, solvent(s) and plasticizer(s) has also been used to create solid electrochromic films. Typically, however, such films do not provide adhesion between the substrates.
Recently, solid plastic electrochromic films prepared by introducing electrochromic molecules and plasticizers into preformed solid thermoplastic polymers have been disclosed. Such films do not require any solvent evaporation or UV polymerization, and can be laminated between two pieces of conductive glass to form electrochromic devices.
Electrochromic compositions comprising an amorphous (co)polymer, an electrochromophore, an ion source, and optionally an electron mediator and a plasticizer have also been disclosed. The electrochromophore comprises a polyalkyleneoxide and an electrochromic moiety.
Nevertheless, there remains a need for an easily manufactured, free-standing electrochromic film that can be laminated between glass or other substrates to create a device that exhibits large changes in light transmission between its “on” and “off” states.